A position sensor is a device that can detect the presence and location of a touch (contact or proximity), by a finger or by an object, such as a stylus, for example, within an area of an external interface of the position sensor. In a touch sensitive display application, the position sensor enables direct interaction with what is displayed on the screen, rather than indirectly with a mouse or touchpad. Position sensors can be attached to or provided as part of devices with a display, including but not limited to computers, personal digital assistants (PDAs), satellite navigation devices, mobile telephones, portable media players, portable game consoles, public information kiosks, and point of sale systems. Position sensors have also been used as control panels on various appliances.
There are a number of different types of position sensors/touch screens, such as resistive touch screens, surface acoustic wave touch screens, capacitive touch screens etc. A capacitive touch screen, for example, may include an insulator, coated with a transparent conductor in a particular pattern. When an object, such as a finger or a stylus, touches or is provided in close proximity to the surface of the screen there is a change in capacitance. This change in capacitance is sent to a controller for processing to determine the position of the touch.
Current touch screen electrode layers are generally made of solid shapes of etched transparent conductive material, such as ITO (indium-tin-oxide), on two layers, forming X and Y electrodes, which define resolution of touch in their respective axis. These layers typically are formed on separate substrates which are then laminated together with a pressure sensitive clear adhesive. This construction method undesirably adds to the overall thickness of the panel while increasing fabrication cost and lowering production yields and reliability.
The electrodes on the layer closest to the display can in some configurations be driven with a low impedance signal which serves to shield the receiving outer receiving electrodes from electrical noise generated by the display itself. However the transparent conductive material has a sheet resistance which is often sufficiently high so as to allow ‘punch through’ of the noise signal from the display, albeit attenuated, to reach the receive electrodes. This punch through of the signal from the display reduces the signal to noise ratio of the touch screen, thereby degrading performance.
The outer, receive electrodes are typically designed to have large surface areas, to reduce losses due to resistance and to facilitate signal pickup from a finger or stylus. However, the use of large surface areas for the receive electrode causes increased common mode noise coupling from a finger when the screen is touched, and it is therefore desirous to minimize this surface area as much as possible. Furthermore, transparent conductive electrodes such as ITO can be expensive, difficult to process and prone to micro-fracturing during processing and application to a panel, and require additional metal traces to them to make electrical connections, which involves additional process steps.